In general, a wind turbine comprises a tower, a nacelle arranged on top of the tower, and a rotor. The rotor comprises a hub and blades, wherein the hub is configured rotatably around a rotational axis particularly corresponding to a longitudinal extension of the nacelle, and the blades are fixed to the hub and extend in a rotational plane being transverse, particularly perpendicular, to a longitudinal hub extension. The hub is connected to an electric generator configured for generating electric energy by e.g. a further rotor shaft and is thus configured for driving the electric generator.
A blade of the rotor of the wind turbine may perform a vibration during an operational or an idle mode of the wind turbine. Such a vibration is characterized by a blade frequency. In particular, a blade frequency may be a blade edge frequency corresponding to a vibration of an edge of the blade, for example a leading blade edge (facing a wind direction) or a trailing edge (pointing in the wind direction).
Knowledge about the blade frequency may allow for detecting a change in blade characteristics and for adapting an operational mode of the wind turbine.
In order to determine the blade edge frequency of one or more blades, a vibration of the blades may be measured using fast responsive sensors, such as accelerometers or strain gauges, which may placed in one or each of the blades of the rotor, and the respective blade edge frequency of the one of each of the blades may be deduced from the measured sensor signals.
It may be further known to use a predetermined blade edge frequency particularly for all blades of the rotor depending on the kind of used blade of the rotor, in order to manually set the blade edge frequency of a blade in a controller of the wind turbine. In this context, it may be assumed that the blade edge frequency of a particular blade may be comparatively constant for a specific class of blades such that the respective blade edge frequency may be selectable from the predetermined blade edge frequencies.
WO 2009/000787 A2 discloses a method for monitoring blade frequencies of each of the blades of a rotor of a wind turbine. Vibrations of a nacelle of the wind turbine as well as an azimuth angle of the blades may be measured. Blade edge frequencies for each of the blades may be determined based on these two quantities.
However, determination of a blade frequency, particular of a blade edge frequency, of one or more blades of a rotor using the above detailed techniques may result in a poor estimation of the blade frequency.